John Gill Commentary Mark 3:16

"and Simon he surnamed Peter;" — Mark 3:16 (ASV)

And Simon he surnamed Peter .
] Or Cephas, which signifies a rock, or stone, because of his courage and constancy, his strength and fortitude, steadiness and firmness of mind: this name was imposed upon him, not at the time of his mission as an apostle; nor when he made that noble confession of his faith in Christ, as the Son of the living God, at which time this name was taken notice of; but when Christ first called him to be his disciple and apostle; see (John 1:42) .